Barrel plating or high-speed rotary plating using a neutral tin plating solution

ABSTRACT

A barrel plating or high-speed rotary plating method for electronic components, and a neutral tin plating solution used therein. A plating solution that includes (A) stannous ions, (B) an acid or a salt, (C) a complexing agent, and (D) a diamine that has a polyoxyalkylene chain, and that has a pH in a range between 4 and 8 is used. The use of this neutral tin plating solution prevents the electronic components from coupling together during the barrel plating, enabling an improvement in manufacturability in barrel plating.

FIELD OF THE INVENTION

The present invention relates to a barrel plating or high-speed rotary plating method for electronic components using a neutral tin plating solution, and to the neutral tin plating solution used therein. More specifically, it relates to a barrel plating or high-speed rotary plating method, and to a neutral tin plating solution used therein, which is capable of improving the coverage of plating onto electronic components while preventing electronic components from coupling to each other when barrel plating or high-speed rotary plating is performed.

BACKGROUND OF THE INVENTION

Tin plating is carried out broadly for the purpose of improving solderability of electronic components. The method for tin plating electronic components can be selected from various different methods, such as barrel plating, rack plating, and the like, depending on the shape, the structure of the plating location, and the like. Barrel plating is generally used as a method for plating small electronic components such as chip resistors, chip capacitors, and the like. Moreover, in recent years, high-speed rotary plating has been carried out through flow-through platers, and the like, as a plating method for small electronic components.

Normally, when performing barrel plating, electronic components that are to be plated, and conductive metal (dummy balls) for improving the conductivity during plating, are loaded into a cage-shaped container that is known as a barrel, and the electronic components are plated through applying an electric current while rotating or vibrating the barrel in a state wherein it is immersed in a plating solution. However, in recent years, as the sizes of electronic components have grown smaller, there has been a tendency for the electronic components to couple to each other, or the dummy balls to clump together, or for the electronic components and the dummy balls to stick together, during barrel plating, producing a problem in that there is an adverse effect on manufacturability of electronic components in the barrel plating. Moreover, depending on the type of plating solution, foaming may occur at the solution surface, producing a problem with an adverse effect on the plating operations. In high-speed rotary plating, the electronic components that are the objects to be plated are inserted into a disk-shaped cell, and plating is performed while the cell is rotated at a high speed, and the same problems can occur as with barrel plating.

Japanese Unexamined Patent Application Publication 2009-299123 discloses a tin electroplating solution for chip components with a pH of 1 or less, including stannous ions, oxygen, dipolyoxyalkylene alkyl amine or amine oxide, and a bonding inhibitor. However, when electronic component ceramic members that are the objects to be plated are inserted, the ceramic members are damaged through the use of a plating solution with a pH of 1 or less, which is not preferred.

Japanese Unexamined Patent Application Publication 2003-293186 discloses a neutral tin plating bath that includes a soluble stannous salt, an acid or salt, a complexing agent selected from an oxycarboxylic acid, or the like, and a quaternary amine polymer. However, in research by the present inventors, a neutral tin plating solution that uses a quaternary amine polymer is not adequately effective in preventing coupling between the objects to be plated, and further improvements are desired.

SUMMARY OF THE INVENTION

Consequently, the object of the present invention is to enable suppression of coupling between the electronic components even when using barrel plating or high-speed rotary plating of electronic components that have been miniaturized in recent years, to provide an electronic component plating method, and a neutral tin plating solution used therein, wherein the foaming of the plating solution is extremely little when performing barrel plating or high-speed rotary plating.

The present inventors arrived at the present invention through discovering that it is possible to improve manufacturability in barrel plating or high-speed rotary plating through preventing the objects being plated from coupling together, and through having little foaming in the plating solution, through the addition, to the neutral tin plating solution, of a diamine that has a polyoxyalkylene chain.

That is, one aspect of the present invention provides an electronic component barrel plating or high-speed rotary plating method that includes a step that carries out barrel plating or high-speed rotary plating on electronic components using a tin plating solution that includes (A) stannous ions, (B) an acid or a salt, (C) a complexing agent, and (D) a diamine that includes a polyoxyalkylene chain, and wherein the pH is in a range between 4 and 8.

Moreover, one aspect of the present invention provides a neutral tin plating solution for barrel plating or high-speed rotary plating that includes (A) stannous ions, (B) an acid or a salt, (C) a complexing agent, and (D) a diamine that includes a polyoxyalkylene chain, and wherein the pH is in a range between 4 and 8.

DETAILED DESCRIPTION OF THE INVENTION

Abbreviations used throughout this Specification have the following meanings, unless defined otherwise: g=grams; mg=milligrams; ° C.=degrees Celsius; min=minutes; m=meters; cm=centimeters; L=liters; mL=milliliters; A=amperes; and dm²=square decimeters. All numeric values include the boundary values thereof, and can be combined in arbitrary sequencing. Throughout the present specification, the terms “plating solution” and “plating bath,” have identical meanings, and can be used interchangeably. Moreover, unless stated specifically, throughout this Specification, percent (%) indicates percent by weight.

The barrel plating or high-speed rotary plating method for electronic components according to the present invention has, as a distinctive feature, the use of a plating solution that includes (A) stannous ions, (B) an acid or a salt, (C) a complexing agent, and (D) a diamine that includes a polyoxyalkylene chain, and wherein the pH is in a range between 4 and 8. This plating solution will be explained sequentially below.

(A) Stannous Ions

In the plating solution used in the present invention, the stannous ions are included as a required structural requirement. Stannous ions are double-oxidized tin ions. In the plating solution, an arbitrary compound may be used insofar as it is a compound that is able to supply stannous ions. Generally, a tin salt of an inorganic acid or an organic acid that is used in a plating solution is preferred. For example, as tin salts of inorganic acids there are stannous salts of sulfuric acid and of hydrochloric acid, and as tin salts of organic acids there are, for example, stannous salts of substituted or non-substituted alkane sulfonic acids and alkanol sulfonic acids, such as, for example, methane sulfonic acid, ethane sulfonic acid, propane sulfonic acid, 2-hydroxy ethane-1-sulfonic acid, 2-hydroxy propane-1-sulfonic acid, and 1-hydroxy propane-2-sulfonic acid. Particularly preferred sources for supplying stannous ions are stannous sulfonate salts for salts of inorganic acids, or stannous methane sulfonate salts for salts of organic acids. These compounds able to supply these ions may be used singly or in mixtures of two or more thereof.

The inclusion proportion of the stannous ions within the plating solution may be, for example, between 5 g/L and 30 g/L, as tin ions, or, preferably, between 8 g/L and 25 g/L, or more preferably, between 10 g/L and 20 g/L.

(B) Acids or Salts

The plating solution used in the present invention can use an arbitrary acid or salt, insofar as the acid or salt has a function that provides conductivity to the plating solution. Any organic acid, inorganic acid, or salt thereof may be used. As organic acids there are, for example, substituted or non-substituted alkane sulfonic acids or alkanol sulfonic acids, such as, for example, methane sulfonic acid, ethane sulfonic acid, propane sulfonic acid, 2-hydroxy ethane-1-sulfonic acid, 2-hydroxy propane-1-sulfonic acid, and 1-hydroxy propane-2 sulfonic acid. Methane sulfonic acid is preferred. For inorganic acids, sulfuric acid or hydrochloric acid, for example, may be used, and sulfuric acid is preferred. As salts thereof, an arbitrary salt may be used, such as an alkaline metal salt, an alkaline earth metal salt, an ammonium salt, an amine salt, or the like. The acids and salts may be used singly or in combinations of two or more. While use of the inorganic acid or organic acid, or salt thereof, used for the stannous ion salt, as described above, is preferred, there is no limitation thereto.

Note that, so that the plating solution that is used in the present invention will be a neutral plating solution, when an organic acid or inorganic acid, as described above, is used for the conductive substance, any of a variety of bases may be added, as a pH adjusting agent, to the plating solution to adjust the pH into a range of between 4 and 8.

The inclusion proportion of the acid or salt in the plating solution is, for example, between 30 g/L and 300 g/L, and preferably between 50 g/L and 200 g/L, and more preferably between 80 g/L and 150 g/L.

(C) Complexing Agent

The plating solution used in the present invention includes a complexing agent as a required structural condition. Typically a double-ionized tin ion in a tin plating solution is stable in a strong acid, but becomes unstable when near neutral, and tends to segregate out as a tin metal, so the plating solution tends to decompose. For the tin plating solution of the present invention to have a pH in a range between 4 and 8, a complexing agent must be included. There is no particular limitation on the complexing agent insofar as it has the effect of stabilizing the plating solution. The complexing agent may be, for example, gluconic acid, citric acid, malonic acid, succinic acid, tartaric acid, or a salt thereof. Of these, gluconic acid or a salt thereof is preferred, and sodium gluconate is most preferred.

The inclusion proportion of the complexing agent in the plating solution is, for example, between 80 g/L and 250 g/L, and preferably between 100 g/L and 200 g/L, and more preferably between 125 g/L and 175 g/L.

(D) Diamene that has a Polyoxyalkylene Chain

The plating solution used in the present invention includes, as a required structural condition, a diamene that has a polyoxyalkylene chain. The inclusion of this compound prevents the objects being plated from coupling together, enabling an improvement in manufacturability in barrel plating. Preferably the diamene that includes a polyoxyalkylene alkylene chain is a compound described by the following chemical formula:

Here, in the equation above, R₁ is a straight-chain or branched-chain alkylene group with a carbon number between 1 and 10, and preferably is a straight-chain or branched-chain alkylene group with a carbon number between 1 and 6, and more preferably is a straight-chain or branched-chain alkylene group with a carbon number between 1 and 4. Each of R₂ through R₅ is, independently, a straight-chain or branched-chain alkylene group with a carbon number between 1 and 10, and preferably is a straight-chain or branched-chain alkylene group with a carbon number between 1 and 6, and more preferably is a straight-chain or branched-chain alkylene group with a carbon number between 1 and 4. Each of n, m, o, and p is an integer between 0 and 15, where n, m, o, and p are not all 0. Preferably, each of n, m, o, and p is an integer between 1 and 12, and more preferably, an integer between 1 and 8. Note that the total of n, m, o, and p is between 1 and 60, and preferably between 2 and 30, and more preferably between 4 and 25.

The present inventors discovered that a plating solution that includes the compounds set forth above is able to reduce coupling of electronic components when performing barrel plating or high-speed rotary plating, and enables plating operations to be performed stably with little production of bubbles that would be a problem when performing plating.

The weight-average molecular weight of the diamene that has the polyoxyalkylene chain is preferably between 200 and 1100, and more preferably between 300 and 600. Here the weight-average molecular weight is a value that is measured using the GPC method.

The diamene with the polyoxyalkylene chain may use that which is commercially available, and, for example, may use Adeka Polyether EDP-450, Adeka Polyether BM-54 (manufactured by Adeka Co., Ltd.), or the like.

The inclusion proportion of the diamene with a polyoxyalkylene chain in the plating solution is, for example, between 0.1 g/L and 30 g/L, and preferably between 0.5 g/L and 20 g/L, and more preferably between 1 g/L and 5 g/L.

(E) pH Adjusting Agent

The pH of the neutral tin plating solution used in the present invention is in a range between 4 and 8, and preferably between 5 and 7. A base or an acid may be added to the plating solution to adjust the pH into this range. Acids that may be used include, for example, methane sulfonic acid, ethane sulfonic acid, propane sulfonic acid, hydrochloric acid, sulfuric acid, and the like. As bases that can be used there are, for example, sodium hydroxide, potassium hydroxide, and aqueous ammonia.

The neutral tin plating solution used in the present invention may include, as other arbitrary components, components that are typically added to plating solutions. For example, oxidation inhibiting agents, brightening agents, smoothing agents, conductive salts, anode solvents, corrosion inhibiting agents, wetting agents, and the like, may be used.

(F) Oxidation Inhibiting Agents

In the plating solution according to the present invention, an oxidation inhibiting agent may be used as appropriate. The oxidation inhibiting agent is used to prevent the tin ions from being oxidized from 2+ to 4+, where, for example, hydroquinone, catechol, resorcin, phloroglucin, pyrogallol, hydroquinone, sulfonic acid, a salt thereof, or the like, may be used.

The oxidation inhibiting agent may be used suitably in a concentration, in the plating bath, of, for example, between 100 mg/L and 50 g/L, or preferably between 200 mg/L and 20 g/L, and more preferably, between 0.5 g/L and 5 g/L.

There is no particular limitation on the sequence with which the various components are added when initially making up the electrolytic plating bath; however, from the point of view of safety, the acids and salts are added after adding water, the tin salt is then added after thorough mixing, after which the other chemicals are added as necessary after thorough dissolution.

There are no particular limitations on the electronic components that are the objects to be plated in the present invention, and they may be, for example, resistors, capacitors, inductors (or inductive transformers), thermistors, varistors, variable resistors, variable capacitors, or other passive components, or crystal oscillators, LC filters, ceramic filters, delay lines, SAW filters, or other functional components, or switches, connectors, relay fuses, optical connectors, or other connecting components. In particular, capacitors, inductors, thermistors, and varistors have ceramic portions within the components, meaning that strong acid plating solutions cannot be used, and thus plating using the neutral plating bath of the present invention is preferred.

Plating Method

A method for carrying out barrel plating using the plating solution according to the present invention will be explained. As described above, normally, when performing barrel plating, the electronic components that are the objects to be plated are loaded together with dummy balls and, in a state wherein they are immersed in a plating solution, an electric current is applied as the barrel is rotated, but the barrel plating method according to the present invention includes the case of placing, into the barrel, only the objects to be plated, without addition of the dummy balls. The barrel plating method may use an arbitrary apparatus, such as of a horizontal or an inclined rotating barrel type, a pivoting barrel type, a vibrating barrel type, or the like. The barrel plating may be carried out with the temperature of the plating solution between, for example, 10 and 50° C., and preferably between 20 and 40° C. Moreover, the cathode current density may be selected as appropriate in a range that is, for example, between 0.01 and 10 A/dm², and preferably between 0.05 and 5 A/dm², and more preferably between 0.1 and 0.5 A/dm². A method may be selected wherein, for example, the plating bath is not stirred during the plating process, or, for example, may be stirred using a stirrer, or the method may have a fluid flow using a pump.

Moreover, when performing high-speed rotary plating using the plating solution according to the present invention, the plating may be carried out using, for example, a flow-through plater, or the like, under conditions of between 10 and 50° C., with a cathode current density between 0.01 and 10 A/dm², with small electronic components, which are the objects to be plated, being plated during high-speed rotation.

Embodiment 1

An electrolytic bath of a tin plating solution with the following composition was prepared:

(A) Stannous Methane Sulfonate: 39 g/L (15 g/L as Tin Ions) (B) Sodium Methane Sulfonate: 100 g/L (C) Sodium Gluconate: 145 g/L (D) Adeka Polyether EDP-450 (Molecular Weight: 450): 2 g/L

(E) Sodium Hydroxide: Enough to cause the pH of the plating solution to be 6

(F) Sodium Erythorbate: 2 g/L (G) Water: Balance Structure of the Adeka Polyether EDP-450

Two liters of the tin plating solution of the electrolytic bath that was prepared were used to carry out barrel tin plating, under the following conditions, on chip resistors that had been subjected to nickel plating, and various evaluations were performed. The results are presented in Table 1.

Barrel Plating Conditions 1

Barrel: Yamamoto Mini Barrel (Capacity: Approximately 140 mL)

Objects Subjected to Plating: Chip Resistors: 5 g

Dummy Balls: Steel Balls, Diameter: 0.71 through 0.85 mm, 60 g

Current Density: 0.2 A/dm²

Plating Time: 50 min

Plating Solution Temperature: 35° C.

Speed of Rotary: 6 rpm

(Solder Wettability Test)

Two liters each of plating solutions according to the embodiments and the comparative examples were prepared, and tin electroplating was carried out for 50 min. with a bath temperature of 35° C. at 0.2 A/dm², to deposit a 5-μm tin plating coating on the external electrodes of the chip resistors. On each of the tin plating coatings produced, a damp heat testing procedure was performed (a PCT procedure with a temperature of 105° C., humidity of 100%, for four hours), and the solder wettability of the plating coating before and after the damp heating testing was evaluated using a multi-solderability tester SWET-2100, manufactured by Tarutin, to measure the zero-cross time (“ZCT”) through a solder paste equilibrium method. The measurement conditions were as follows:

Zero-Cross Time Measurement Conditions

-   -   Solder Paste: Sn:Ag:Cu=95.75:3.5:0.75     -   Test Temperature: 245° C.     -   Immersion Depth: 0.15 mm     -   Test Speed: 2 mm/s     -   Holding Time: 8 s

The solder wettability was measured both before and after the PCT process, and the results are given in Table 1.

The coupling proportions (the proportion by weight of the adhered chips, relative to the total chip weight), and the clumping proportions (the proportion by weight of the dummy balls that were clumped together, relative to the total weight of dummy balls) were calculated, and, similarly, are given in Table 1.

External Appearance of Plating and Particle Size (Grain Size) of Plating

Hull cell testing was carried out using the plating solutions of the electrolytic baths that were prepared, and the external appearances of the plating coatings that were produced were observed with the naked eye, and the plating grain sizes were measured using SEM (at 2000×).

Comparative Example 1

An electrolytic bath of a tin plating solution was prepared with the same composition as in the first embodiment, except for the use of 2 g/L of lauryl dimethylaminoacetate betaine (Amphitol 20BS, manufactured by Kao Corp.) instead of the (D) Adeka Polyether EDP-450 in the first embodiment. The foamability of the plating solution was high, and thus unsuitable for barrel plating.

Comparative Examples 2 to 4

An electrolytic bath of a tin plating solution was prepared with the same composition as in the first embodiment, except for the use of 2 g/L of a quaternary ammonium salt polymer (Papiogen P-113 (Comparative Example 2)), SenkaFix 401 (Comparative Example 3), and a quaternary ammonium salt (Eretat M-65 (Comparative Example 4)), respectively, instead of the (D) Adeka Polyether EDP-450 in the first embodiment. The plating solution produced in Comparative Example 4 had high foamability, so was not suitable for barrel plating. For the plating solutions produced in Comparative Example 2 and Comparative Example 3, the same procedures were performed for each as in the first embodiment. The external appearance of the plating coating obtained in the Hull cell testing for the plating solution produced in Comparative Example 2 was white, with an excellent result, but in the barrel plating test the coupling rate was high, at 81.3%. For the plating solution obtained in Comparative Example 3, the external appearance of the plating coating produced in the Hull cell testing was grayish-black, which was undesirable.

TABLE 1 Embodiment Comparative Example 1 1 2 3 4 (D) Adeka 2 — — — — component Polyether (d/L) EDP-450 Anphitol — 2 — — — 20BS Papiogen — 2  — — P-113 SenkaFix 401 — — — 2 — Eretat M-65 — — — — 2 Foamability Low High Low Low High External Okay — Okay No Good — Appearance (White) (White) (Grayish Black) Grain size 2 through 7 — 5 through 10 — — (μm) Coupling Chip 25.0 — 81.3 — Rate (%) Clumping Dummy Balls 2.4 —  2.62 — — Rate (%) Solder Zero-Cross 1.87/1.93 — — — — Wettability Time (Units: Test Minutes)

Embodiment 2

Testing was carried out in the same way as in the first embodiment, with the exception of changing Barrel Plating Conditions 1 to Barrel Plating Conditions 2, below, and the coupling rate and clumping rate were measured. The result was that the chip coupling rate was 0.1%, and the dummy ball clumping rate was 0%. Moreover, there was little foaming of the plating solution.

Barrel Plating Conditions 2

Barrel: Yamamoto Mini Barrel (Capacity: Approximately 140 mL)

Objects Subjected to Plating: Chip Resistors: 5 g

Dummy Balls: Steel Balls, Diameter: 0.71 through 0.85 mm, 60 g

Current Density: 0.2 A/dm²

Plating Time: 50 min

Plating Solution Temperature: 35° C.

Speed of Rotary: 12 rpm 

What is claimed is:
 1. A barrel plating or high-speed rotary plating method for electronic components, including a step for barrel plating or high-speed rotary plating of an electronic component using a tin plating solution that includes (A) stannous ions, (B) an acid or a salt, (C) a complexing agent, and (D) a diamine that has a polyoxyalkylene chain, and wherein the pH is in a range between 4 and
 8. 2. The method as set forth in claim 1, wherein barrel plating is carried out with a current density between 0.05 and
 0. 3. The method as set forth in claim 1, wherein the electronic component is selected from a capacitor, an inductor, a thermistor, and a varistor.
 4. A neutral tin plating solution for barrel plating or high-speed rotary plating comprising: (A) stannous ions; (B) an acid or a salt; (C) a complexing agent; and (D) a diamine having a polyoxyalkylene chain, wherein a pH is in a range between 4 and
 8. 5. The neutral tin plating solution for barrel plating or high-speed rotary plating as set forth in claim 4, wherein the diamine having the polyoxyalkylene chain has a molecular weight between 200 and
 1100. 